(a) Field of the Invention
This invention relates to a direct-injection spark ignition engine for directly injecting a fuel into a combustion chamber in a cylinder and igniting a mixture thus prepared in a condition of being stratified around an electrode of a spark plug, and more particularly relates to a technique of controlling behavior of fuel spray for promoting suitable stratification of the mixture by effectively using a tumble in the combustion chamber.
(b) Description of the Prior Art
Conventionally, direct-injection spark ignition engines of such type are constructed so that a high-pressure fuel injection nozzle is disposed so as to be presented to a combustion chamber in a cylinder and a cavity of predetermined shape is formed in a piston crown surface, whereby a fuel injected from the fuel injection nozzle is first brought into impingement against an inner wall surface or a bottom surface of the cavity opposed thereto and then confined in the cavity for stratification around a spark plug electrode.
There also exist engines of a type which concentrates a mixture into a cavity or transports it to the vicinity of a spark plug electrode using in-cylinder airflows, such as a swirl flow or a tumble flow, in a combustion chamber. For example, an in-cylinder injection type engine, as disclosed in Japanese Unexamined Patent Publication Gazette No. 11-141338, reverses the direction of fuel injection from that of the tumble, pushes back the mixture which tends to overflow from the cavity to confine it in the cavity by the action of the tumble, and transports the mixture to the vicinity of the spark plug electrode in the cavity while promoting vaporization of fuel droplets or mixture thereof with air.
Alternatively, like an in-cylinder injection type spark ignition engine disclosed in Japanese Unexamined Patent Publication Gazette No. 11-200866, there exist engines of a type which transports a fuel having been spread into a cavity toward a spark plug so as to carry the fuel with a tumble while suppressing adhesion of the fuel to a piston crown surface by forming the cavity substantially in the center of the piston crown surface and into a spherical shape to enhance retentivity of the tumble flow and by widening the spray cone angle of spray from a fuel injection nozzle up to, for example, 70xc2x0-90xc2x0 to damp fuel spray penetration.
However, it is difficult to say that any prior art engines mentioned above suitably stratify the mixture over a wide range of operating conditions different in load and revolving speed. When viewed over a complete range of engine operating conditions, they still have plenty of room to enhance effects such as fuel economy improvement by stratified-charge combustion operation. Specifically, in the engine which requires to confine the mixture in the cavity like the former prior art (Japanese Unexamined Patent Publication No. 11-141338), the range of engine operating conditions within which the mixture can be suitably stratified is strictly constrained by the cavity size and shape, and in fact the range of operating conditions within which the engine is capable of stratified operation is limited to a narrow region on the low-load and low-speed end of its control map. Therefore, the engine can provide only a small effect of fuel economy improvement.
Furthermore, in such a direct-injection engine, since the inner wall surface of the cavity against which fuel spray injected from the fuel injection nozzle impinges is generally located in the vicinity of the center line of the cylinder, it cannot be avoided that the inner wall surface inhibits the growth of a flame core in the initial combustion stage and decreases flame propagation performance, which in fact deteriorates combustion quality. In addition, in this engine, since the fuel spray is caused to impinge against the inner wall surface or the bottom surface of the cavity, the amount of adhesion of the fuel to the wall surface and the like is increased. This invites inconveniences of decrease in fuel economy and increase in unburnt hydrocarbon (HC).
For example, FIG. 35 comparatively shows results of a test conducted as follows: a plurality of pistons different in their cavity shapes were prepared and the rate of fuel economy improvement and the rate of power output improvement of each engine by direct injection were experimentally obtained. According to the figure, there is a so-called xe2x80x9ctrade-off relationshipxe2x80x9d between fuel economy improvement and power output improvement. In the engine (Point A of the figure) in which the cavity in the shape of a deep dish is provided like the former prior art, the effect of fuel economy improvement at low load and low speeds is enhanced because the fuel spray can be confined suitably for stratification, whereas the effect of power output improvement is deteriorated because of degradation in the combustion quality particularly at higher engine speeds.
In a so-called xe2x80x9cflat pistonxe2x80x9d (Point C) in which the crown surface thereof is simply formed in a concave shape, the effect of power output improvement at higher engine speeds is enhanced, whereas the effect of fuel economy improvement is inevitably deteriorated because of the difficulty in suitably stratifying the mixture at low load. As an intermediate between the above two types, there is an engine (Point B) in which the inner wall surface of the cavity opposed to the fuel injection nozzle is largely inclined. However, it cannot be expected that the engine of this type largely improves both fuel economy and power output.
Next, consider the latter prior art (Japanese Unexamined Patent Publication No. 11-200866). It seems that the latter prior art also produces the same inconveniences as caused in the former prior art when its cavity size is small. On the other hand, if its cavity size is large, then suitable stratification of the mixture becomes difficult like the above-mentioned flat piston. Specifically, the engine of this type intends to transport the fuel having been dispersed into the cavity to the spark plug so as to carry it with a tumble. However, when the cavity size is increased, it becomes difficult to concentrate the fuel thus dispersed. And even if the fuel can be concentrated, the fuel goes along with the tumble and passes by the vicinity of the spark plug electrode. Therefore, the period during which the mixture can be ignited by the spark plug is extremely short. Accordingly, it cannot be said that the engine implements suitable mixture stratification.
The present invention has been made in view of the foregoing points and a major object thereof is to improve fuel economy and power output through improvement of combustion quality and extension of a stratified-charge combustion zone by controlling fuel spray behavior in a combustion chamber to allow suitable mixture stratification over a wide range of operating conditions of a direct-injection spark ignition engine when the engine conducts stratified-charge combustion operation.
To attain the above object, in solutions of the present invention, a tumble is generated to flow from the vicinity of a spark plug electrode toward a fuel injection nozzle at the compression stroke of a cylinder during stratified-charge combustion operation of an engine, and a fuel is injected at a suitable penetration so as to go against the tumble so that a flammable mixture can be retained in the vicinity of the spark plug at an exact ignition timing for the cylinder.
More specifically, the invention of claim 1 is directed to a direct-injection spark ignition engine in which a spark plug is disposed in a cylinder at a ceiling of a combustion chamber opposed to a crown surface of a piston, a fuel injection nozzle is disposed in the combustion chamber to inject a fuel from a peripheral portion of the combustion chamber, and the fuel injected from the fuel injection nozzle is stratified around an electrode of the spark plug during stratified-charge combustion operation. The engine has a configuration which comprises: tumble generating means capable of generating a tumble which flows between the electrode of the spark plug and the crown surface of the piston toward the fuel injection nozzle during a compression stroke of the cylinder; and fuel injection control means for causing the fuel injection nozzle to inject the fuel in correspondence with an ignition timing for the cylinder so that fuel spray from the fuel injection nozzle goes against the tumble and stays in the vicinity of the electrode of the spark plug in the form of a flammable mixture at the time of ignition of the spark plug.
With the above configuration, during the stratified-charge combustion operation of the engine, a tumble generated by the tumble generating means flows toward the fuel injection nozzle at the compression stroke of the cylinder and in this state the fuel injection control means conducts operation control of the fuel injection nozzle in correspondence with an ignition timing for the cylinder. The fuel is thus injected from the fuel injection nozzle to impinge against the tumble. This fuel spray impinges against the tumble to promote atomization and dispersion of the fuel or mixture thereof with the ambient air, and gradually decreases its traveling speed by going against the tumble to stay in the vicinity of the electrode of the spark plug in the form of a flammable mixture of suitable concentration. In this manner, fuel spray behavior is controlled by the tumble so that the mixture can be suitably stratified around the spark plug electrode.
According to such mixture stratification, it is not necessary to cause the fuel spray to impinge against the piston crown surface. Therefore, the amount of adhesion of the fuel to the piston can be largely reduced. Furthermore, since such a cavity as used in the prior art (Japanese Unexamined Patent Publication No. 11-141338) is not necessarily required, this makes it possible to eliminate combustion inhibition of the inner wall surface of the cavity. Accordingly, combustion quality can be improved resulting in enhanced fuel economy and power output performance.
Furthermore, since the present invention has no constraint from the cavity size and shape as the prior art has, it can provide well stratified-charge combustion at higher engine speeds. This enables large fuel economy improvement when viewed over a complete range of engine operating conditions. In addition, since the mixture of suitable concentration can be retained in the vicinity of the spark plug electrode at the ignition timing for the cylinder as mentioned above, the period during which the mixture can be ignited with stability can be extremely elongated. This provides greater flexibility in controlling the ignition timing for the cylinder thereby allowing improvement of fuel economy and power output.
In the invention of claim 2, the electrode of the spark plug is disposed at the midpoint between a first partial flow of the tumble flowing along the crown surface of the piston and a second partial flow of the tumble flowing along the ceiling of the combustion chamber at a certain point after the start timing of fuel injection during the compression stroke of the cylinder and before the ignition timing for the cylinder. With this arrangement, the spark plug electrode is located in the vicinity of the center of the tumble vortex and therefore put into a condition that is hard to affect both the first and second partial tumble flows, i.e., a condition that is easy to retain the mixture. Further, since the spark plug electrode is located away from both the ceiling of the combustion chamber and the crown surface of the piston, this provides well flame propagation performance.
In the invention of claim 3, the electrode of the spark plug is disposed closer to the crown surface of the piston than to the nozzle hole of the fuel injection nozzle when viewed in a direction orthogonal to the center line of the cylinder. With this arrangement, the position of the spark plug electrode is specified and the same operations and effects as obtained in the invention of claim 2 can be obtained.
In the invention of claim 4, the electrode of the spark plug protrudes from the ceiling of the combustion chamber in a direction parallel to the center line of the cylinder, and the distance e between the ceiling of the combustion chamber and the electrode of the spark plug is set at a value satisfying the relationship of exe2x89xa70.4d where d is the distance on the center line of the cylinder between the ceiling of the combustion chamber and the crown surface of the piston when the cylinder is at the top dead center during the compression stroke thereof. With this arrangement, the position of the spark plug electrode is specified and the same operations and effects as obtained in the invention of claim 2 can be obtained.
In the invention of claim 5, for the engine of claim 4, the distance e between the ceiling of the combustion chamber and the electrode of the spark plug is set at a value satisfying the relationship of exe2x89xa70.2d where d is the distance on the center line of the cylinder between the ceiling of the combustion chamber and the crown surface of the piston when the cylinder is at a 55xc2x0 crank angle (CA) before the top dead center during the compression stroke thereof. With this arrangement, the position of the spark plug electrode is specified and the same operations and effects as obtained in the invention of claim 2 can be obtained.
In the invention of claim 6, the electrode of the spark plug in the invention of claim 4 is disposed closer to the ceiling of the combustion chamber than to the crown surface of the piston, when viewed along the center line of a geometrical area of fuel spray from the fuel injection nozzle during the compression stroke of the cylinder, with respect to the center line of fuel spray. In this case, the geometrical area of fuel spray means an area of fuel spray droplets when it is assumed that there is no in-cylinder airflow in the combustion chamber. In this invention, since the electrode of the spark plug is disposed closer to the ceiling of the combustion chamber with respect to the center line of the geometrical area of fuel spray, this suppresses adhesion of large fuel droplets included in initial fuel spray to the electrode thereby obviating the occurrence of smolder in the spark plug.
In the invention of claim 7, the spray cone angle of fuel spray from the fuel injection nozzle during the compression stroke of the cylinder is at a value within the range of approximately 20xc2x0 to 60xc2x0.
If the spray cone angle of fuel spray from the fuel injection nozzle is larger, the fuel spray is more diffused by impingement against the tumble so that the mixture tends to become leaner. On the other hand, if the spray cone angle of spray is too small, carburetion of fuel droplets and mixture thereof with air cannot sufficiently be made, so that the mixture may be excessively rich. To cope with these circumstances, in this invention, the spray cone angle of fuel spray during the compression stroke of the cylinder is set at a value within the range of approximately 20xc2x0 to 60xc2x0. As a result, the mixture residing in the vicinity of the spark plug electrode becomes suitable in concentration thereby ensuring excellent ignition stability.
In the invention of claim 8, the fuel injection nozzle is disposed so that the fuel spray impinges substantially oppositely against the tumble flowing along the crown surface of the piston during the compression stroke of the cylinder.
With this arrangement, since the fuel spray from the fuel injection nozzle impinges substantially oppositely against the tumble, the tumble precisely regulates the traveling speed of the fuel spray thereby retaining the flammable mixture in the vicinity of the spark plug electrode with certainty. Further, since the relative speed of the tumble to the fuel spray is increased, this promotes fuel carburetion.
In the invention of claim 9, the crown surface of the piston is formed with a cavity longer in a direction in which the center line of fuel spray extends when viewed along the center line of the cylinder, and the cavity has such a form that maximizes the distance thereof along the center line of the cylinder from the ceiling of the combustion chamber at the position corresponding to the center line of the cylinder.
With this arrangement, the tumble flows toward the fuel spray along the cavity of the piston crown surface. Accordingly, the tumble can impinge against the fuel spray with stability. Further, since the volume of the combustion chamber can be ensured in the vicinity of the spark plug electrode, the mixture can be easily retained there.
In the invention of claim 10, the deepest portion of the cavity in the invention of claim 9 is positioned in correspondence with the center line of the cylinder. With this arrangement, the tumble can smoothly flow along the cavity and can be retained without collapsing up to the late stage of the compression stroke of the cylinder.
In the invention of claim 11, the deepest portion of the cavity in the invention of claim 9 is positioned closer to an intake side than the position corresponding to the center line of the cylinder. In general, when the tumble in the combustion chamber is weak, there occurs a phenomenon that its partial flow at an exhaust side of the combustion chamber toward the piston crown surface is damped under the influence of upward motion of the piston so that the center of the tumble vortex gradually moves closer to the exhaust side. This results in inconveniences of difficulty in stable impingement of the tumble against the fuel spray and early collapse of the tumble.
On the contrary, in this invention, the cavity of the piston crown surface is formed so that the deepest portion thereof is positioned closer to the intake side than a position corresponding to the center line of the cylinder. Accordingly, the intake side volume of the combustion chamber becomes larger than the exhaust side one thereof so that vortex retentivity at the intake side of the combustion chamber can be enhanced. This suppresses movement of the vortex center to the exhaust side thereby obviating the above inconveniences.
In the invention of claim 12, the crown surface of the piston is formed with a cavity that accommodates the electrode of the spark plug when viewed along the center line of the cylinder, and is also formed with squish area sections each for generating a squish flowing toward the inside of the cavity in cooperation with the opposed ceiling of the combustion chamber, the squish area sections being formed in an outer portion of the piston crown surface, located outside of the cavity, at at least both lateral locations thereof from the electrode of the spark plug with respect to the center line of the fuel spray when viewed along the center line of the cylinder.
With this arrangement, after the middle stage of the compression stroke of the cylinder, a squish area is formed between the squish area sections located on at least both lateral sides of the spark plug electrode in the outer portion of the piston crown surface and the opposed ceiling of the combustion chamber. Squishes are generated to flow toward the inside of the cavity from the squish area. The squishes suppress lateral diffusion of the flammable mixture residing in the vicinity of the spark plug electrode. This makes it possible to form a compact layer of flammable mixture thereby enhancing ignition stability and combustion quality and improving fuel economy.
In the invention of claim 13, for the engine in the invention of claim 12, the opening width of the cavity in the lateral direction when viewed along the center line of the cylinder is maximized in the vicinity of the electrode of the spark plug. With this arrangement, both side walls of the cavity are spaced apart from the spark plug electrode as far as possible. This avoids the side walls from interfering with the growth of the flame core generated in the vicinity of the electrode and deteriorating the flame propagation performance.
In the invention of claim 14, for the engine of claim 13, the opening width of the cavity in the lateral direction in the vicinity of the electrode of the spark plug when viewed along the center line of the cylinder is set to include a geometrical area of fuel spray from the fuel injection nozzle during the compression stroke of the cylinder. With this arrangement, the opening width of the cavity in the lateral direction is ensured sufficiently largely thereby obtaining operations and effects of the invention of claim 13 with certainty.
In the invention of claim 15, the ceiling of the combustion chamber in the invention of claim 13 is formed with a pair of intake ports, and the opening width of the cavity in the lateral direction in the vicinity of the electrode of the spark plug when viewed along the center line of the cylinder is equal to or more than the center distance between the pair of intake ports. With this arrangement, the opening width of the cavity in the lateral direction is ensured sufficiently largely thereby obtaining operations and effects of the invention of claim 13 with certainty.
In the invention of claim 16, the squish area sections in the invention of claim 12 are provided in the outer portion of the crown surface of the piston to continue from both the lateral locations thereof, which interpose the electrode of the spark plug therebetween, toward the exhaust side.
With this arrangement, a squish area can be formed in the outer portion of the piston crown surface over a wider region including both lateral sides of the spark plug electrode. Accordingly, squishes flowing from the squish area toward the inside of the cavity can enwrap the flow of the mixture and the tumble as a whole. As a result, a compact layer of flammable mixture can be formed in the vicinity of the spark plug electrode, thereby further enhancing the operations and effects of the invention of claim 12.
In the invention of claim 17, the crown surface of the piston is formed with a squish area section for generating squishes in cooperation with the ceiling of the combustion chamber so that a layer of the flammable mixture residing in the vicinity of the electrode of the spark plug takes on a diametrically diminished form on the side closer to the ceiling of the combustion chamber when viewed along the center line of the fuel spray.
With this arrangement, after the middle stage of the compression stroke of the cylinder, a squish area is formed between the squish area section in the piston crown surface and the opposed ceiling of the combustion chamber, and squishes are generated to flow toward the inside of the cavity therefrom. The squishes cause the layer of the flammable mixture residing in the vicinity of the spark plug electrode to take on a diametrically diminished form on the side closer to the ceiling of the combustion chamber when viewed along the center line of the fuel spray. Consequently, the mixture can be concentrated in the vicinity of the spark plug electrode through the squishes thereby forming a compact layer of flammable mixture. This improves ignition stability and combustion quality like the invention of claim 12.
In the invention of claim 18, the crown surface of the piston in the invention of claim 17 is formed with a cavity so as to accommodate the electrode of the spark plug and the layer of flammable mixture residing in the vicinity of the electrode of the spark plug when viewed along the center line of the cylinder, and the squishes act to suppress diffusion of the layer of flammable mixture so that the profile of the layer of flammable mixture is spaced apart from the side walls of the cavity and has a larger distance from each of the side walls as it approaches the ceiling of the combustion chamber.
With this configuration, the above operations and effects as obtained in the invention of claim 17 can be obtained. In addition, since the profile of the layer of flammable mixture residing in the vicinity of the electrode of the spark plug is spaced apart from the side walls of the cavity, this prevents adhesion of the fuel to the side walls and suppresses deterioration of fuel economy and increase of unburnt hydrocarbon in the exhaust gas due to adhesion of fuel.
In the invention of claim 19, the fuel injection control means controls penetration of fuel spray through the fuel injection nozzle in accordance with the flow rate of the tumble opposed to the fuel spray. With this configuration, even if the flow rate of the tumble in the combustion chamber of the cylinder changes, the penetration of the fuel spray can be controlled in accordance with the change in the tumble flow rate so that the flammable mixture can be retained in the vicinity of the spark plug electrode. Accordingly, well stratified-charge combustion can be implemented with stability even if the engine operating conditions change.
In the invention of claim 20, the fuel injection control means in the invention of claim 19 controls the penetration of fuel spray from the fuel injection nozzle in accordance with the revolving speed of a crank shaft. In general, the flow rate of the tumble in the combustion chamber of the cylinder changes with the revolving speed of the crank shaft of the engine (hereinafter, also referred to the engine revolving speed). Therefore, if the penetration of the fuel spray is controlled in accordance with the revolving speed of the crank shaft, it can be controlled in accordance with the flow rate of the tumble.
In the invention of claim 21, for the engine in the invention of claim 19, injection pressure regulating means is provided for regulating the fuel injection pressure of the fuel injection nozzle, and the fuel injection control means is arranged to cause the injection pressure regulating means to increase the fuel injection pressure in increasing the penetration of the fuel spray and to decrease the fuel injection pressure in decreasing the penetration of the fuel spray.
With this configuration, the penetration of the fuel spray can be surely controlled by changing the injection pressure of fuel from the fuel injection nozzle through the injection pressure regulating means. Further, since regulation of the fuel injection pressure is excellent in controllability and responsibility, it also has a high responsibility to change in the engine operating conditions.
In the invention of claim 22, the fuel injection control means in the invention of claim 21 is arranged to correctively control the operation of the injection pressure regulating means in accordance with the temperature conditions of the combustion chamber so that the fuel injection pressure becomes larger as the temperature of the combustion chamber is increased even if the fuel injection quantity and the revolving speed of the crank shaft are substantially constant.
The penetration of the fuel spray through the fuel injection nozzle changes depending upon the temperature conditions of the combustion chamber. The temperature conditions of the combustion chamber change if the fuel injection quantity (engine load) or the revolving speed of the crank shaft (engine revolving speed) varies, and it also changes depending upon the warming-up conditions of the engine or the existence/absence of recirculation of exhaust gas. As an example, after the warming up of the engine, the temperature of the intake air sucked in the combustion chamber becomes higher as compared with that before the completion of the warming up, and the combustion temperature is also increased after the warming up so that the temperature of the combustion chamber becomes higher. Furthermore, the exhaust temperature becomes higher with the increase in the combustion temperature. At the recirculation of the exhaust gas, the intake temperature is also increased under the influence of the high-temperature exhaust gas. As a result, fuel carburetion is promoted so that the penetration of fuel spray shows a tendency to decrease.
To cope with the above problem, in this invention, the fuel injection pressure is corrected in accordance with the temperature conditions of the combustion chamber even if the revolving speed and load conditions of the engine are constant. Accordingly, variation in penetration of fuel spray can be prevented and fuel spray behavior can be controlled with stability.
In the invention of claim 23, the fuel injection nozzle in the invention of claim 19 is provided with a variable spray angle mechanism for adjusting the spray cone angle of fuel spray, and the fuel injection control means is arranged to cause the variable spray angle mechanism to decrease the spray cone angle of fuel spray in increasing the penetration of fuel spray and to increase the spray cone angle of fuel spray in decreasing the penetration of fuel spray. In this manner, the penetration of fuel spray can be surely controlled by changing the spray cone angle of fuel spray of the fuel injection nozzle through the variable spray angle mechanism.
In the invention of claim 24, the fuel injection control means in the invention of claim 19 is arranged to increase the penetration of fuel spray through the fuel injection nozzle to correspond to rise in the revolving speed of the crank shaft until the revolving speed of the crank shaft reaches a predetermined value and suppress increase of the penetration when the revolving speed of the crank shaft reaches or exceeds the predetermined value. In addition, the engine is further provided with: variable tumble means for making the flow rate of the tumble variable; and tumble control means for operating the variable tumble means to suppress increase in the flow rate of the tumble which corresponds to increase in the revolving speed of the crank shaft when the revolving speed of the crank shaft reaches or exceeds the predetermined value.
When the revolving speed of the crank shaft of the engine is increased so that the flow rate of the tumble reaches or exceeds the predetermined value, if the fuel penetration is increased correspondingly, impingement between the fuel spray and the tumble become too much hard. This results in an inconvenience that the mixture is diffused to the surroundings so that the degree of stratification is decreased. To cope with this problem, in this invention, such a revolving speed as the impingement becomes much hard is experimentally obtained and predetermined, and when the revolving speed of the engine reaches or exceeds the predetermined value, increase in the flow rate of the tumble and increase in the penetration of fuel spray are suppressed. In this manner, over-diffusion of the mixture as mentioned above can be prevented.
In the invention of claim 25, the variable tumble means in the invention of claim 24 comprises an intake airflow control valve for changing the flowing conditions of the intake air flowing into the combustion chamber. With this configuration, the flow rate of the tumble in the combustion chamber can be surely controlled by changing the flowing conditions of the intake air flowing into the combustion chamber through the intake airflow control valve.
In the invention of claim 26, the variable tumble means in the invention of claim 24 is a variable valve timing mechanism for changing the valve timing of at least one of intake and exhaust valves. With this configuration, the flow rate of the tumble in the combustion chamber can be surely controlled by changing the valve timing of at least one of the intake valve and the exhaust valve through the variable valve timing mechanism.
In the invention of claim 27, the tumble control means in the invention of claim 24 correctively controls the operation of the variable tumble means in accordance with the temperature conditions of the combustion chamber so that the flow rate of the tumble is lower as the temperature of the combustion chamber is higher even if the fuel injection quantity and the revolving speed of the crank shaft are substantially constant.
The penetration of the fuel spray through the fuel injection nozzle changes depending upon the temperature conditions of the combustion chamber. The temperature conditions of the combustion chamber change if the fuel injection quantity (engine load) or the revolving speed of the crank shaft (engine revolving speed) varies, and it also changes depending upon the warming-up conditions of the engine or the existence/absence of recirculation of exhaust gas. As an example, after the warming up of the engine, the temperature of the intake air sucked in the combustion chamber becomes higher as compared with that before the completion of the warming up, and the combustion temperature is also increased after the warming up so that the temperature of the combustion chamber becomes higher. Furthermore, the exhaust temperature becomes higher with the increase in the combustion temperature. At the recirculation of the exhaust gas, the intake temperature is also increased under the influence of the high-temperature exhaust gas. As a result, fuel carburetion is promoted so that the penetration of fuel spray shows a tendency to decrease.
To cope with the above problem, in this invention, the flow rate of the tumble which is balanced against the penetration of the fuel spray is corrected in accordance with the temperature conditions of the combustion chamber even if the revolving speed and load conditions of the engine are constant. Accordingly, even if the penetration of fuel spray varies as mentioned above, the attendant adverse effects can be eliminated and fuel spray behavior can be controlled with stability.
Next, the invention of claim 28 is directed to a direct-injection spark ignition engine in which a spark plug is disposed in a cylinder at a ceiling of a combustion chamber opposed to a crown surface of a piston, a fuel injection nozzle is disposed in the combustion chamber to inject a fuel from a peripheral portion of the combustion chamber, and the fuel injected from the fuel injection nozzle is stratified around an electrode of the spark plug during stratified-charge combustion operation. In this engine, the spark plug is disposed so that the electrode thereof protrudes from the ceiling of the combustion chamber in a direction parallel to the center line of the cylinder and the distance e between the ceiling of the combustion chamber and the electrode thereof has a value satisfying the relationship of exe2x89xa70.4d where d is the distance on the center line of the cylinder between the ceiling of the combustion chamber and the crown surface of the piston when the cylinder is at the top dead center during the compression stroke thereof. Further, the fuel injection nozzle is disposed so that the spray cone angle of fuel spray therefrom has a value within the range of approximately 20xc2x0 to 60xc2x0 and the center line of fuel spray therefrom has a tilt angle within the range of approximately 25xc2x0 to 40xc2x0 with respect to an assumed plane orthogonal to the center line of the cylinder. Furthermore, the engine has a configuration which comprises: tumble generating means capable of generating a tumble which flows between the electrode of the spark plug and the crown surface of the piston toward the fuel injection nozzle during a compression stroke of the cylinder; and fuel injection control means for controlling penetration of fuel spray through the fuel injection nozzle in accordance with the flow rate of the tumble and causing the fuel injection nozzle to inject fuel against the tumble.
With the above configuration, like the invention of claim 1, during the stratified-charge combustion operation of the engine, the fuel injection control means causes the fuel injection nozzle to inject fuel spray against a tumble flowing from the center toward the periphery of the combustion chamber during the compression stroke of the cylinder and controls penetration of the fuel spray in accordance with the tumble intensity. In this manner, fuel spray behavior is controlled by the tumble so that the mixture can be suitably stratified around the electrode of the spark plug. Accordingly, the stratified-charge combustion zone of the engine can be enlarged resulting in largely improved fuel economy. In addition, since the mixture can be retained around the spark plug electrode, this provides greater flexibility in controlling the ignition timing for the cylinder thereby allowing improvement of fuel economy and power output.
In this case, since the center line of fuel spray from the fuel injection nozzle has a tilt angle within the range of approximately 25xc2x0 to 40xc2x0 with respect to an assumed plane orthogonal to the center line of the cylinder and the spray cone angle of fuel spray therefrom has a value within the range of approximately 20xc2x0 to 60xc2x0, the fuel spray effectively impinges against the tumble to be approximately opposite it without substantially adhering to the piston crown surface. Accordingly, the tumble sufficiently promotes fuel carburetion and precisely controls fuel spray behavior so that the concentration of the mixture residing in the vicinity of the spark plug electrode can be made extremely suitable.
Further, since the electrode of the spark plug protrudes from the ceiling of the combustion chamber and is located in the vicinity of the center of the tumble vortex, the retention period of the mixture around the electrode can be elongated, which further increases the above-mentioned flexibility in controlling the ignition timing and improves propagation performance of flame front from a flame core generated in the vicinity of the electrode. This also provides further improvement of fuel economy and power output.
In the invention of claim 29, the engine in the invention of claim 28 further comprises: variable tumble means for making the flow rate of the tumble variable; and tumble control means for operating the variable tumble means so that a tumble ratio during the compression stroke of the cylinder falls within the range of approximately 1.1 to 2.3.
When the penetration of fuel spray is controlled in accordance with the flow rate of the tumble by the fuel injection control means in the invention of claim 28, if they are too much great even though balanced one against another, the fuel spray is diffused to the surroundings by their hard impingement so that suitable stratification of the mixture cannot be provided. To cope with this problem, in this invention, the operation of the variable tumble means is controlled by the tumble control means so that the tumble ratio in the combustion chamber is held within the range of approximately 1.1 to 2.3. In this manner, over-diffusion of the fuel spray as mentioned above can be prevented like the invention of claim 21.
In the invention of claim 30, the engine in the invention of claim 28 further comprises injection pressure regulating means for regulating the fuel injection pressure of the fuel injection nozzle, and the fuel injection control means controls the operation of the injection pressure regulating means so that the fuel injection pressure falls within the range of approximately 3 MPa to 13 MPa.
When the penetration of fuel spray is controlled in accordance with the flow rate of the tumble by the fuel injection control means in the invention of claim 28, if they are too much great even though balanced one against another, the fuel spray is diffused to the surroundings by their hard impingement so that suitable stratification of the mixture cannot be provided. To cope with this problem, in this invention, the injection pressure of fuel from the fuel injection nozzle is regulated by the injection pressure regulating means to fall within the range of approximately 3 MPa to 13 MPa. In this manner, over-diffusion of the fuel spray as mentioned above can be prevented while atomization property and other properties of fuel are well maintained within the range where the flow rate of the tumble is not excessively high.
As described so far, according to the direct-injection spark ignition engine of the invention of claim 1, during its stratified-charge combustion operation, fuel spray behavior is controlled by a tumble without confining fuel having been injected by the fuel injection nozzle at the compression stroke of the cylinder to the cavity or the like of the piston. As a result, the mixture around the spark plug can be suitably stratified regardless of the engine operating conditions. This improves combustion quality thereby providing enhanced fuel economy and power output performance, and extends the stratified-charge combustion zone thereby enabling large improvement of fuel economy when viewed over a complete range of engine operating conditions. In addition, since the mixture can be retained in the vicinity of the spark plug electrode, flexibility in controlling the ignition timing is enhanced. This also enables improvement of fuel economy and power output performance.
According to the invention as set forth in each of claims 2 to 5, since the position of the spark plug electrode is optimized, this enhances ignition stability and other properties of the mixture thereby sufficiently exhibiting the effects of the invention of claim 1.
According to the invention of claim 6, since the electrode of the spark plug is positioned apart from the center line of the geometrical area of fuel spray mainly from the fuel injection nozzle, this suppresses adhesion of large fuel droplets to the electrode thereby obviating the occurrence of smolder of the spark plug.
According to the invention of claim 7, since the spray cone angle of fuel spray through the fuel injection nozzle is optimized, the mixture in the vicinity of the spark plug electrode can be optimized in its concentration conditions thereby sufficiently exhibiting the effects of the invention of claim 1.
According to the invention of claim 8, since the direction of fuel spray from the fuel injection nozzle is optimized, fuel spray behavior can be effectively controlled by the tumble while carburetion of the fuel spray is promoted. This sufficiently exhibits the effects of the invention of claim 1.
According to the invention of claim 9, since the crown surface of the piston is formed with a cavity of suitable form, the tumble can impinge against the fuel spray with stability and the mixture can be easily retained in the vicinity of the spark plug electrode. This sufficiently exhibits the effects of the invention of claim 1.
According to the invention of claim 10, the sectional form of the cavity becomes suitable and therefore the retentivity of the tumble can be enhanced.
According to the invention of claim 11, the effect of upward motion of the piston can be reduced even if the tumble in the combustion chamber is relatively weak. This enhances retentivity of the tumble.
According to the invention of claim 12, squishes flowing toward the spark plug electrode from both lateral sides with respect to the center line of fuel spray suppress diffusion of the flammable mixture residing in the vicinity of the spark plug electrode, thereby enhancing ignition stability and combustion quality.
According to the invention as set forth in each of claims 13 to 15, since inner walls of the cavity are spaced apart from the spark plug electrode as far as possible, this avoids the inner walls from interfering with the growth of the flame core and deteriorating flame propagation performance.
According to the invention of claim 16, squishes, which flow from the squish area toward the inside of the cavity to enwrap the mixture flow and the like, allow for the formation of a compact layer of flammable mixture in the vicinity of the spark plug electrode, thereby further enhancing the effects of the invention of claim 12.
According to the invention of claim 17, squishes allow for the formation of a compact layer of flammable mixture taking on a diametrically diminished form on the side closer to the ceiling of the combustion chamber in the vicinity of the spark plug electrode. This improves ignition stability and combustion quality like the invention of claim 12.
According to the invention of claim 18, the effects as obtained in the invention of claim 17 can be obtained. In addition, it can be prevented that fuel adheres to the side walls of the cavity provided in the piston crown surface. This suppresses deterioration of fuel economy and increase of unburnt hydrocarbon in the exhaust gas.
According to the invention of claim 19, since the penetration of fuel spray through the fuel injection nozzle is controlled in accordance with the flow rate of the tumble opposed to the fuel spray, the mixture can be suitably stratified by controlling fuel spray behavior through the tumble even if the engine operating conditions change.
According to the invention of claim 20, since the penetration of fuel spray through the fuel injection nozzle is controlled in accordance with the revolving speed of a crank shaft, the penetration of the fuel spray can be controlled in accordance with the flow rate of the tumble.
According to the invention of claim 21, the penetration of the fuel spray can be surely controlled by changing the injection pressure of fuel from the fuel injection nozzle through the injection pressure regulating means.
According to the invention of claim 22, even if the temperature conditions of the combustion chamber change, fuel spray behavior can be controlled with stability by correcting the fuel injection pressure in accordance with the temperature conditions of the combustion chamber.
According to the invention of claim 23, the penetration of fuel spray can be surely controlled by changing the spray cone angle of fuel spray from the fuel injection nozzle through the variable spray angle mechanism.
According to the invention of claim 24, over-diffusion of the mixture can be prevented by suppressing both increase in the flow rate of the tumble and increase in the penetration of fuel spray before impingement between the fuel spray and the tumble become too much hard.
According to the invention of claim 25, the flow rate of the tumble in the combustion chamber can be surely controlled by changing the flowing conditions of the intake air flowing into the combustion chamber through the intake airflow control valve.
According to the invention of claim 26, the flow rate of the tumble in the combustion chamber can be surely controlled by changing the valve timing of at least one of the intake valve and the exhaust valve through the variable valve timing mechanism.
According to the invention of claim 27, even if the temperature conditions of the combustion chamber change, the attendant variation in the penetration of fuel spray can be cancelled out by correcting the flow rate of the tumble, which is balanced against the penetration of the fuel spray, in accordance with the temperature conditions of the combustion chamber. Accordingly, behavior of fuel spray can be controlled with stability.
According to the direct-injection spark ignition engine of the invention of claim 28, like the invention of claim 1, well stratified-charge combustion can be provided thereby improving fuel economy and power output, the stratified-charge combustion zone can be enlarged thereby enabling large improvement of fuel economy, and flexibility in controlling the ignition timing can be enhanced thereby further improving fuel economy and power output performance. Furthermore, since the direction and spray cone angle of fuel spray from the fuel injection nozzle and the position of the spark plug electrode are optimized, this prevents adhesion of fuel to the piston crown surface and provides further improvement in combustion quality thereby improving fuel economy and power output performance as much as possible.
According to the invention of claim 29, since the tumble ratio in the combustion chamber of the engine cylinder is set within the range of approximately 1.1 to 2.3, over-diffusion of the fuel spray can be prevented thereby improving ignition stability.
According to the invention of claim 30, since the fuel injection pressure of the fuel injection nozzle is set within the range of approximately 3 MPa to 13 MPa, this prevents over-diffusion of the fuel spray for the improvement in ignition stability while maintaining well atomization property and other properties of fuel.